The optical appearance of objects or of the surfaces thereof, particularly surfaces on motor vehicles, is greatly determined by the surface properties thereof. However, since the human eye is suitable only to a limited extent for the objective determination of surface properties, there is a need for aids and apparatuses for the qualitative and quantitative determination of surface properties.
Examples of such surface properties are the gloss, orange peel, colour, macrostructure or microstructure, image sharpness, haze, surface structure and/or surface topography and the like.
Furthermore, coatings which contain so-called effect pigments are enjoying greater popularity in recent times. In these coatings, a large number of effect pigments which act like small mirrors are arranged in the paint layer. Ideally, such effect pigments have a planar surface and are arranged essentially parallel to the coating itself.
In reality, however, the surface of such effect pigments is not planar but rather may be curved for example in a concave or convex manner. As a result of this curvature, a light beam impinging on these pigments is widened or narrowed and this leads to an altered optical appearance of the surface as a whole. If such a curved effect pigment reflects radiation impinging on it, then the viewing angles at which this effect pigment can be perceived depends inter alia on the curvature of the effect pigment. This angular range will be referred to below as the angular lifetime. Other unevennesses, such as cracks, edges or in general any topographical surface defects, may also have an effect on the angular lifetime.
The object of the present invention is to achieve a more accurate objective examination of such effect pigments. This object is achieved by a method and apparatus for examining surface properties, comprising the following steps:                emitting radiation at a first predefined emission angle onto a surface to be examined;        receiving at a first reception angle at least part of the radiation emitted at the first emission angle and thrown back from the surface to be examined, and outputting a plurality of first measured values which are characteristic of the received radiation;        emitting radiation at a second predefined emission angle onto a surface to be examined;        receiving at a second reception angle at least part of the radiation emitted at the second emission angle and thrown back from the surface to be examined, and outputting a plurality of second measured values which are characteristic of the received radiation, wherein at least the first and second emission angles or the first and second reception angles are different; and        carrying out a comparison between the first measured values and the second measured values.        
In the method according to the invention for examining surface properties, in a first method step radiation is emitted at a first predefined emission angle onto a surface to be examined. Furthermore, at least part of the radiation emitted at the first emission angle and thrown back from the surface to be examined is received at a first reception angle, and a plurality of first measured values are output which are characteristic of the received radiation.
In a further method step, radiation is emitted at a second predefined emission angle onto the surface to be examined, and at least part of the radiation emitted at the second emission angle and thrown back from the surface to be examined is received at a second reception angle, and a plurality of second measured values are output which are characteristic of the received radiation.
According to the invention, at least the emission angles or the reception angles are different. In a further method step, a comparison is carried out between the first measured values and the second measured values.
Radiation which is thrown back will be understood to mean any radiation, in particular reflected and/or scattered radiation, which passes from the surface onto a radiation detector device. With particular preference, the radiation is light and particularly preferably light in the visible wavelength range.
The radiation that is thrown back may be composed of reflected and scattered fractions, in particular of scattered light from the surface itself and of reflected light from the individual effect pigments. Preferably, the surface to be examined is imaged onto a radiation detector device or else imaging optics are used.
Preferably, either the emission angle or the reception angle is retained and the respective other angle is changed. In this way, an observation of the surface under at least two different angles can be carried out. The emission angles and the reception angles will be defined below as angles relative to a central perpendicular line from the surface. Preferably, the plurality of measured values is an array which characterises the radiation impinging on a detector device.
If, for example, the radiation is emitted twice at the same emission angle and is received at different reception angles, it can be checked whether the recordings at different reception angles point to a specific effect pigment. The angular lifetime of this effect pigment can thus be deduced from an angular difference between these two reception angles. Preferably, the radiation is emitted at a plurality of emission angles and the radiation that is thrown back is received at one specific reception angle. Conversely, it is also possible for radiation to be emitted at just one specific emission angle and to be received at a plurality of reception angles. Finally, a plurality of emission angles and a plurality of reception angles may be used.
By virtue of this plurality of angles, it is possible to specify with a high degree of accuracy the angular range at which a given effect pigment still reflects light. In this way, a very accurate image of the curvature of these effect pigments can be obtained. Preferably, the measurement does not take place in reflection, i.e. the emission angles and the reception angles are not equal but opposite.
In addition, scatter properties or reflection properties of the effect pigments can also be examined, and in particular the extent to which specific effect pigments act as mirrors or as scattering bodies.
In a further preferred method, the radiation is received in a spatially resolved manner. In this case, for example, a radiation detector with a CCD chip is provided, which outputs a spatially resolved image of the impinging radiation. Preferably, the difference between the different emission angles or reception angles is less than 5°, preferably less than 3°, particularly preferably less than 1° and particularly preferably less than 0.5°. In this way, the curvature of the effect pigment in question can be determined in a very precise manner, and the effects of such curved effect pigments can be characterised in a very precise manner.
Preferably, at least one movable radiation detector device is used to receive the radiation thrown back from the surface and in particular the scattered radiation. This radiation detector device can be displaced over a certain angular range in order in this way to determine the angular lifetime of the individual effect pigments. In a further preferred embodiment, at least one movable radiation device is used to emit the radiation onto the surface. Here, too, the radiation device can particularly preferably be moved in the circumferential direction so that, in this way, an effect pigment can be illuminated from different angles in order thus to determine the curvature thereof. In addition, besides the curvature of the individual effect pigments, it is also possible to determine the orientation thereof with respect to the plane of the coating.
However, it is also possible to use a plurality of radiation devices which are arranged at different angles with respect to the surface, and to use on the other hand a plurality of radiation detector devices which are respectively spaced apart from one another by a small angular distance. Conversely, it is also possible to use a plurality of radiation devices which are arranged at a small angular distance from one another, and to use on the other hand a plurality of radiation detector devices which are arranged at a larger angular distance from one another, for example at a distance of 10°.
The present invention also relates to an apparatus for examining surface properties. This apparatus comprises a first radiation device which emits radiation at a first predefined emission angle onto a surface to be examined. Also provided is a first radiation detector device which receives at a first reception angle at least part of the radiation emitted onto the surface and thrown back from the latter and outputs a plurality of first measured values which are characteristic of the radiation emitted at the first emission angle and received at the first reception angle.
According to the invention, measurement means are provided which allow an emission of the radiation at a second emission angle and the reception at a second reception angle of the radiation thrown back, wherein the measurement means allow the outputting of a plurality of second measured values which are characteristic of the radiation emitted at the second emission angle and received at the second reception angle, wherein at least the two emission angles or the two reception angles are different. In this case, preferably both the radiation detector device and the measurement means allow a spatially resolved reception of the radiation thrown back from the surface.
Also provided is a comparison device which compares the first measured values with the second measured values. Again in the case of this apparatus, it can be checked whether a specific effect pigment is still perceived at different illumination and viewing angles, and thus the angular lifetime of this effect pigment can be deduced.
A comparison device is understood to mean any device which allows a comparison of at least two values. In the simplest case, this may be a display device which outputs the first group of measured values and the second group of measured values to the user, so that the latter can carry out comparisons. Preferably, however, the comparison device performs these comparisons at least partially automatically. This may be achieved for example if the abovementioned arrays of measured values are loaded into a memory and aligned with one another and then individual measured values or individual groups of measured values are compared with one another. It can thus be checked whether certain phenomena are present, such as the appearance of certain effect pigments in the different sets of measured values. Preferably, the sets of measured values are also stored with information relating to this respective beam path, that is to say in particular the respective emission and reception angles. In this way, the angular lifetime of the individual effect pigments can be deduced directly by comparing the individual sets of measured values.
It should be pointed out that the complete information regarding the angular lifetime cannot be obtained until a plurality of recordings have been carried out. In principle, there are various embodiments for configuring said measurement angles. These embodiments will be explained below on the basis of several examples.
Preferably, the measurement means comprise a second radiation device which emits radiation at the predefined second emission angle α2 onto the surface to be examined. In this case, therefore, the two emission angles differ and the reception angles are preferably the same.
In a further preferred embodiment, a plurality of second radiation devices or in general a plurality of radiation devices are provided which direct the radiation onto the surface to be examined. In this way, the surface is illuminated at a plurality of angles which are preferably spaced apart from one another by a small distance, and the radiation that is thrown back is observed at one specific reception angle. Preferably, the surface is illuminated by the individual radiation devices one after the other, in order in this way to avoid any temporal overlap of the measurement values respectively obtained.
Conversely, however, it is also possible to provide a plurality of radiation detector devices which receive at different predefined second viewing angles the radiation thrown back from the surface. In this way, too, the angular lifetime of the individual effect pigments can be determined.
In a further preferred embodiment, the measurement means comprise an emission angle change device which moves the first radiation device relative to the surface and in this way changes the emission angle. In this way, a predefined angular range can be scanned and thus the angular lifetime of the effect pigments can be determined. In this case, the radiation device can preferably be moved in an angular range which also allows the detection of large curvatures. Here, it is possible to change the emission angle in steps of predefined magnitude, wherein these steps may be less than 5°, preferably less than 3°, preferably less than 1° and preferably less than 0.5°.
Conversely, in a further preferred embodiment, a reception angle change device may also be provided which moves the first radiation detector device relative to the surface and in this way changes the reception angle. Both embodiments, i.e. on the one hand a displacement of the radiation device and on the other hand a displacement of the radiation detector device, can be used in the same way to determine the angular lifetime.
Preferably, the first emission angle and the second emission angle differ by less than 5°, preferably by less than 3° and particularly preferably by less than 2°. Here, however, the distance between the radiation devices and the surface is also critical.
In a further preferred embodiment, the first reception angle and the second reception angle differ from one another by less than 5°, preferably by less than 3° and particularly preferably by less than 2°.
In a further preferred embodiment, the measurement means comprise a displaceable diaphragm device. In this case, for example, the diaphragm device can be used to change the emission angle onto the surface, but it can also be used to shift the viewing angle.